The presence of iron oxides as particulate matter in the coolant water of boiling water reactor (BWR) plants has adverse effects on the plant operating characteristics. High concentrations of iron input are known to cause heat transfer problems with the fuel. Also, the activation and transport of Co-60 isotope is related to the iron concentration in the reactor water.
A major hazard in water-cooled nuclear reactors is the accumulation of radioactive substances in the structural portions of the reactor. The buildup of radioactive nuclides occurs on the inner surfaces of components which are in contact with the reactor water. This includes both the primary recirculation circuit and the reactor water cleanup system. During reactor shutdown, workers are exposed to radiation emanating from stainless steel internal walls and inner surfaces of piping. Radioactive materials retained in oxide films which have accumulated on wall and piping surfaces are a major source of radiation exposure. The radioactivity has been found to be predominantly due to the Co-60 isotope. As a result, a substantial effort has been made to identify the key parameters which affect Co-60 buildup and to determine and implement methods for limiting that buildup.
The radiation buildup, controlled mainly by the Co-60 isotope concentration, occurs by two processes. First, the Co-60 isotope which is dissolved in the reactor water incorporates into the crystalline structure of the oxide film as the latter is formed on the stainless steel surfaces. Second, the Co-60 isotope sorbs onto the surfaces of particulates, such as iron oxides, floating in the reactor water or on the fuel. Iron oxide particles which contain sorbed Co-60 isotope tend to deposit in regions of relatively low water flow velocity. This leads to regions of higher radioactivity which are commonly referred to as "hot spots".
The use of very dilute (trace) concentrations of zinc oxide in the reactor water has been demonstrated, both in the laboratory and in boiling water reactors, to limit the incorporation of .sup.60 Co into the oxide film. Because naturally occurring zinc contains .sup.64 Zn isotope which is converted to radioactive .sup.65 Zn in a nuclear reactor, the .sup.64 Zn is removed during the manufacturing process. Zinc from which the .sup.64 Zn isotope has been substantially removed is referred to as "depleted zinc".
The adverse effect of iron oxide particulate matter in the reactor coolant is amplified by the use of depleted zinc to reduce the uptake of Co-60 on out-of-core piping surfaces. A high concentration of iron requires an increase in the amount of depleted zinc injected and a concomitant increase in cost. Hence, it is desirable to develop a technique to reduce particulate iron in the BWR coolant. Understanding the form and nature of particulate iron at the various types of plants is very important information when considering solutions to mitigate the problem.
The major iron oxide forms found in BWR coolant are magnetite (Fe.sub.3 O.sub.4) and hematite (Fe.sub.2 O.sub.3), which are magnetic species, and .alpha.--FeOOH and .gamma.--FeOOH, which are non-magnetic species. This determination is conventionally made by accumulating particulate samples on filter membranes and doing a laboratory analysis for crystal structure by X-ray diffraction. The size distribution of each species has been difficult to determine, even using scanning electron microscopy (SEM). There has also been a serious concern that the collected material may change form during transport to the analytical laboratory.
Thus, there is a need for a particle sizing system which measures the distribution of particles in a flowing water stream in a relatively short time period (a few minutes). This information is useful in the design of filters for removing the particulate matter as well as for identifying the source of the particulate matter.